Electric discharge machine is an indispensable machine method nowadays. Its basic principle is explained briefly as follows. The problems occurred at present also will be discussed.
In principle, electric discharge machine is a thermal machine method that converts electric energy to thermal energy of work pieces and rapidly melts the work pieces. In other words, in electric discharge machine an electric arc discharging phenomenon occurs through a machine liquid in a very close gap formed between the electrode and the work piece, and a heating effect takes place on the work piece to result in melting of the work piece. The work piece forms an electric discharge trace due to electric discharging. The process is repeatedly performed to fabricate a product forming a desired shape.
Refer to FIG. 1 for a conventional wire cut electric discharge machine apparatus. First, an ignition power supply device 11 provides the required power and connects to an upper machine guide 12 and a lower machine guide 13 in a parallel connection, and also connects to a work piece 14. A cutting wire 16 has two ends connecting respectively to the upper machine guide 12 and the lower machine guide 13 to establish electric connection. The upper machine guide 12 (or the lower machine guide 13), work piece 14 and cutting wire 16 form an ignition circuit. A main power supply device 15 is located between the upper machine guide 12 (or lower machine guide 13) and the work piece 14 to form a discharge circuit. An ignition switch 17 is provided to activate the ignition power supply device 11, for supplying electric power. Through the ignition circuit, an electric arc is generated between the cutting wire 16 and the work piece 14. Once the electric arc occurs, the ignition switch 17 is turned off and a main discharge switch 18 is activated. Through the main discharge circuit, the main power supply device 15 provides electric power, to produce an electric discharge machine phenomenon between the cutting wire 16 and the work piece 14. As a result, the work piece 14 generates heat and melts partially to form a required shape. Finally the main discharge switch 18 is turned off and pauses for a selected time period.
Refer to FIG. 2 for the time sequence of a conventional wire cut electric discharge apparatus. As shown in the drawing, electric power is supplied concurrently to the upper machine guide and the lower machine guide. The ignition time A of the upper machine guide and the ignition time B of the lower machine guide are synchronously activated at the ignition time E1. The discharge time C of the upper machine guide and the discharge time D of the lower machine guide also synchronously occur at discharge time E2. Then the process is stopped for a preset off time E3. After the work piece is insulated from the cutting wire, the foregoing machine process is repeated. Operation energy is simultaneously generated from the upper machine guide and the lower machine guide as follows:E1=(11+12)×Vgwhere E1 is energy, 11 is the electric current of the upper machine guide, 12 is the electric current of the lower machine guide, Vg is the voltage in the gap between the work piece and the cutting wire. Discharge power is as follows: P1=E1×F1where P1 is the discharge power, F1 is the discharge frequency.
In conventional techniques for high speed machine, the detected discharge frequency is about 80 kHz, if the off time is set at 8 us. As every discharge must be followed by a waiting off time to recovery the discharge gap insulating again, shortening the off time can effectively increase the discharge frequency and improve machine efficiency.